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Link Scheduling & Queuing

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Link Scheduling & Queuing

COS 461: Computer Networks

http://www.cs.princeton.edu/courses/archive/spr14/cos461/

- Link scheduling (not covered on Monday)
- Queuing practice questions
- Miscellanea

- First-in first-out scheduling
- Simple, but restrictive

- Example: two kinds of traffic
- Voice over IP needs low delay
- E-mail is not that sensitive about delay

- Voice traffic waits behind e-mail

3

- Multiple levels of priority
- Always transmit high-priority traffic when present

- Isolation for the high-priority traffic
- Almost like it has a dedicated link
- Except for (small) delay for transmission

- Almost like it has a dedicated link
- Possible starvation

High

Low

4

- Each queue gets a fraction of the link bandwidth
- Rotate across queues on a small time scale
- What if a queue is empty during its time slice?
- Move on to next queue if work conserving

50% red, 25% blue, 25% green

5

- If non-work conserving
- Flows get at most their allocated weight

- If work-conserving
- Send extra traffic from one queue if others are idle
- Bytes, not packets
- Higher or lower utilization than non-work conserving?

- Results in max-min fairness
- Maximize the minimum rate of each flow

6

- FIFO
- One queue, trivial scheduler

- Strict priority
- One queue per priority level, simple scheduler

- Weighted fair scheduling
- One queue per class, and more complex scheduler

7

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

- Drop-tail solves the full queue problem T/F

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

- Drop-tail solves the full queue problem T/F

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

- Drop-tail solves the full queue problem T/F
- Random Early Detection (RED) solves the full queue problem T/F

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

- Drop-tail solves the full queue problem T/F
- Random Early Detection (RED) solves the full queue problem T/F

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

- Drop-tail solves the full queue problem T/F
- Random Early Detection (RED) solves the full queue problem T/F
- RED solves the lockout problem for TCP flows T/F

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

- Drop-tail solves the full queue problem T/F
- Random Early Detection (RED) solves the full queue problem T/F
- RED solves the lockout problem for TCP flows T/F

1 Drop-tail has fewer burst losses than RED

T/F

1 Drop-tail has fewer burst losses than RED

T/F

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F

3 RED drops every incoming packet with some probability P > 0

T/F

1 Drop-tail has fewer burst losses than RED

T/F

T/F

3 RED drops every incoming packet with some probability P > 0

T/F

- Receive window
- flow control

- Congestion window
- Additive increase / multiplicative decrease
- Triple duplicate ACKs
- Slow-start, fast retransmit, fast recovery, etc.